1. Field of the Invention
The present invention relates to a control system for controlling an electric power generating apparatus on a hybrid vehicle.
2. Description of the Related Art
Electric vehicles propelled by a propulsive electric motor which is powered by a battery include hybrid vehicles which carry an electric power generating apparatus including an electric generator powered by an internal combustion engine for improving the mileage of the vehicle and increasing the service life of the battery. While the hybrid vehicle is running, the electric generator is operated by the internal combustion engine to output electric power which is supplied to charge the battery.
It is customary for such hybrid vehicles to detect, from time to time, the remaining electric energy stored in the battery and control the internal combustion engine or the electric generator to produce an amount of electric power depending on the detected remaining electric energy stored in the battery.
If the amount of electric power generated by the electric generator is controlled depending on only the remaining electric energy stored in the battery, however, the battery tends to be excessively charged at the time the vehicle runs at a lower speed when the propulsive electric motor consumes a smaller amount of electric power, and the battery tends to be insufficiently charged at the time the vehicle runs at a higher speed when the propulsive electric motor consumes a greater amount of electric power because the electric generator generates the same amount of electric power in either case insofar as the remaining electric energy stored in the battery is the same. As a consequence, the electric generator cannot generate electric power to charge the battery efficiently in a manner to make up for the consumption of stored electric energy by the battery in various running conditions of the vehicle.
The inventors have proposed an electric power generation control system for detecting, from time to time, the remaining electric energy stored in a battery and the speed of a hybrid vehicle and controlling the electric power generated by an electric generator depending on the remaining electric energy stored in the battery and the speed of the vehicle which have been detected, as disclosed in Japanese patent applications Nos. 6-197071 and 6-197072, for example. According to the proposed electric power generation control system, a target electric power to be outputted by the electric generator and a target rotational speed to be achieved by an internal combustion engine to operate the electric generator are determined from predetermined maps and tables depending on the remaining electric energy stored in the battery and the speed of the vehicle which have been detected. The current of the electric generator and the throttle opening of the internal combustion engine are controlled in order to achieve the target electric power and the target rotational speed.
The proposed electric power generation control system allows the electric generator to generate electric power to charge the battery efficiently in a manner to make up for the consumption of stored electric energy by the battery depending on the remaining amount of electric energy stored in the battery and in various running conditions of the vehicle.
With the proposed electric power generation control system, however, when the vehicle runs at frequently varying speeds, a target electric power to be outputted by the electric generator and a target rotational speed to be achieved by the internal combustion engine are determined exactly depending on such frequently varying speeds, so that the actual electric power outputted by the electric generator and the actual rotational speed of the internal combustion engine are subject to frequent variations. Accordingly, when the vehicle runs at frequently varying speeds, the load on the internal combustion engine and the rotational speed thereof are liable to vary, resulting in limitations posed on efforts to increase the emission and vibration control capabilities of the internal combustion engine.